Polymethylmethacrylate (PMMA) bone cements are based on the pioneering work of Sir Charnley (Charnley, J.: Anchorage of the femoral head prosthesis of the shaft of the femur. J. Bone Joint Surg. 42 (1960) 28-30.). PMMA bone cements consist of a liquid monomer component and a powder component. The monomer component generally contains the monomer, methylmethacrylate, and an activator (N,N-dimethyl-p-toluidine) dissolved therein. The powder component, which is also referred to as bone cement powder, comprises one or more polymers, a radiopaquer, and the initiator dibenzoylperoxide. The polymers of the powder component are produced on the basis of methylmethacrylate and comonomers, such as styrene, methylacrylate or similar monomers by means of polymerisation, preferably by suspension polymerisation. During the mixing of powder component and monomer component, swelling of the polymers of the powder component in the methylmethacrylate generates a dough that can be shaped plastically and is the actual bone cement. During the mixing of powder component and monomer component, the activator, N,N-dimethyl-p-toluidine, reacts with dibenzoylperoxide while forming radicals. The radicals thus formed trigger the radical polymerisation of the methylmethacrylate. Upon advancing polymerisation of the methylmethacrylate, the viscosity of the cement dough increases until the cement dough solidifies.
Methylmethacrylate is the monomer used most commonly in polymethylmethacrylate bone cements. Redox initiator systems usually consist of peroxides, accelerators and, if applicable, suitable reducing agents. Radicals are formed only if all ingredients of the redox initiator systems act in concert. For this reason, the ingredients of the redox initiator system in the separate starting components are arranged appropriately such that these cannot trigger a radical polymerisation. The starting components are stable during storage provided their composition is adequate. Only when the two starting components are mixed to produce a cement dough, the ingredients of the redox initiator system, previously stored separately in the two pastes, liquids or powders react with each other forming radicals which trigger the radical polymerisation of the at least one monomer. The radical polymerisation then leads to the formation of polymers while consuming the monomer, whereby the cement dough is cured.
PMMA bone cements can be mixed by mixing the cement powder and the monomer liquid in suitable mixing beakers with the aid of spatulas. It is a disadvantage of this procedure that air inclusions may arise and/or be present in the cement dough thus formed and may have an adverse influence on the mechanical properties of the cured bone cement and may therefore cause destabilisation of the bone cement later on.
A multitude of vacuum cementing systems has been proposed to prevent air inclusions in bone cement dough, some of which are listed in the following for exemplary purposes: U.S. Pat. No. 6,033,105 A, U.S. Pat. No. 5,624,184 A, U.S. Pat. No. 4,671,263 A, U.S. Pat. No. 4,973,168 A, U.S. Pat. No. 5,100,241 A, WO 99/67015 A1, EP 1 020 167 A2, U.S. Pat. No. 5,586,821 A, EP 1 016 452 A2, DE 36 40 279 A1, WO 94/26403 A1, EP 1 005 901 A2, U.S. Pat. No. 5,344,332 A. In the vacuum cementing systems thus specified, there is a need to connect an external vacuum pump to generate the negative pressure. These are generally operated by compressed air utilising the Venturi principle. The compressed air required for operation of the vacuum pumps is supplied either by stationary compressed air facilities or by electrically-operated compressors. In addition, it is also feasible to use electrically-operated vacuum pumps to generate vacuum.
Cementing systems, in which both the cement powder and the monomer liquid are already packed in separate compartments of the mixing systems and are mixed with each other in the cementing system only right before application of the cement, are a development of cementing technology. Said closed full-prepacked mixing systems were proposed through EP 0 692 229 A1, DE 10 2009 031 178 B3, U.S. Pat. No. 5,997,544 A, U.S. Pat. No. 6,709,149 B1, DE 698 12 726 T2, and U.S. Pat. No. 5,588,745 A. Said mixing systems also require an external vacuum source.
Patent DE 10 2009 031 178 B3 discloses a generic mixing system having a two-part dispensing plunger for closure of a cement cartridge. A combination of a gas-permeable sterilisation plunger and a gas-impermeable sealing plunger is used in this context. This principle of a closed vacuum mixing system is implemented in the closed cementing system, PALACOS® PRO, made and distributed by Heraeus Medical GmbH.
WO 00/35506 A1 proposes a device, in which the polymethylmethacrylate bone cement powder is stored in a cartridge, whereby the cement powder fills the entire volume of the cartridge and the volume of the intervening spaces between the particles of the cement powder correspond to the volume of the monomer liquid required for production of bone cement dough with the cement powder stored in the cartridge. The design of said device is such that the monomer liquid is supplied into the cartridge from above by the action of a vacuum, whereby a vacuum is applied to a vacuum connector on the underside of the cartridge for this purpose. As a result, the monomer liquid is drawn through the cement powder, whereby the air situated in the intervening space of the cement particles is replaced by the monomer liquid. This does not include a mechanical mixing of the cement dough thus formed by means of a stirrer.
It is a disadvantage of this system that cement powders that swell rapidly in the presence of the monomer liquid cannot be mixed with this device because the rapidly swelling cement powder particles form a gel-like barrier of approximately 1 to 2 cm after ingress of the monomer liquid into the cement powder and impede the migration of the monomer liquid through the entire cement powder. Moreover, exposed to the action of a vacuum, it cannot be excluded that the monomer liquid is aspirated through the vacuum connector after the monomer liquid fully penetrates into the cement powder. In this case, an insufficient amount of monomer liquid is available for curing by means of radical polymerisation and/or the mixing ratio, and therefore the consistency of the bone cement, is changed inadvertently. Moreover, it is a problem that the air enclosed between the cement powder particles is to be displaced from top to bottom by the monomer liquid, because the air, having a lower specific weight as compared to the monomer liquid, tends to migrate upwards in the cement powder and not downwards in the direction of the vacuum connector.
If vacuum mixing systems are used for cementing, external vacuum pumps need to be provided. Said vacuum pumps are expensive and need to be cleaned after use. Moreover, vacuum hoses for connecting the vacuum pumps to the vacuum mixing systems are required. Said vacuum hoses need to be enclosed with the vacuum mixing systems. Accordingly, prior to the mixing using a vacuum mixing system, the vacuum pump needs to be set-up in the surgical theatre (OR) and must be connected to an energy source, such as compressed air or electrical power. Then, the vacuum pump is connected to the vacuum mixing system by means of a vacuum hose. Said installation steps take up costly OR time and are potentially error-prone. The vacuum pump and connecting conduits to the vacuum mixing system and to external energy sources and supply conduits take up space and are potential tripping hazards and stumbling blocks that can disturb the often hectic procedure during a surgery.
An interesting concept has been proposed through EP 1 886 647 A1. Here, the cement powder is stored in an evacuated cartridge and the monomer liquid is situated in a separate container. The cartridge, which is kept at a negative pressure, being opened causes the monomer liquid to be aspirated into the cartridge without any ingress of air. A bone cement dough free of air inclusions is thus produced. Said concept requires the cartridge to remain closed in vacuum-tight manner during the storage before use such that no non-sterile air can enter into the cartridge. For this purpose, the cartridge must be sealed in a stable hermetic manner. Accordingly, one associated disadvantage is that the design is quite elaborate and that the content of the cartridge cannot be mixed by an externally-operated mixing system after aspiration of the monomer liquid since a feed-through for a mixing rod or a mixing tube would not readily be permanently vacuum-tight. All full-prepacked mixing systems known to date utilise a vacuum or a negative pressure to transfer the monomer liquid into the cement powder.
Accordingly, it is the object of the invention to overcome the disadvantages of the prior art. Specifically, the disadvantages of the known vacuum mixing systems having an external vacuum source are to be overcome. One of the objects of the invention is to develop a simple, closed device, in which polymethylmethacrylate bone cement powder (cement powder) and monomer liquid can be stored in separate compartments and can be mixed subsequently. It shall be possible for medical users to combine and mix the polymethylmethacrylate bone cement powder and the monomer liquid inside the device without the medical user being exposed to either of the two cement components. Any contact of the medical user with the polymethylmethacrylate bone cement powder and with the monomer liquid shall be excluded to the extent possible. The device to be developed is a full-prepacked mixing system. The device shall be designed appropriately such that the monomer liquid can be transferred into the polymethylmethacrylate bone cement powder without the use of external vacuum pumps driven by compressed air or compressors. Moreover, it is important that the device is functional in the absence of external energy sources, such as compressed air, vacuum or electrical current, even under the simplest external conditions and that it reliably ensures the production of bone cement dough. It shall be possible to use the device autonomously without additional technical equipment.
It is another object of the invention to provide a device, which makes it possible to specifically control the volume of monomer liquid that is transferred into the cement dough, such that the ratio of the volume of monomer liquid to the amount of cement powder can be varied in order to control the consistency and thus the processing properties of the bone cement.
Moreover, a method is to be provided that enables a monomer transfer and a mixing in full-prepacked mixing systems. In this context, the mixing system to be developed shall be manufactured mainly from inexpensive plastics.
Moreover, a device that is inexpensive to manufacture and working reliably for the mixing of a medical cement and, if applicable, for storage of the starting components of the cement, and a method for the mixing of the bone cement is to be devised, in which a simple manual operation can be used to mix the starting components, if possible without having to use an external or additional energy source and without air inclusions arising in the mixing material.
The main component of the polymethylmethacrylate bone cement, as mixing material, shall be a powder and the second component shall be present in the form of a liquid. Preferably, it shall be possible to store the two starting components of the bone cement separate from each other in the full-prepacked mixing system and to combine them safely through the use of the device.